Thin film transistors are used in metal oxide semiconductor SRAM fabrication processes and are also frequently used in flat panel liquid crystal displays, as well as many other applications.
Thin film transistors are formed using a variety of different processes. A typical process is illustrated in FIGS. 1A-1D. A conduction layer, typically a layer of highly doped polysilicon, is deposited and lithographically defined as a gate electrode 12 on the substrate 11 which typically is a silicon wafer which may have bulk transistors, interconnect layers and contact holes in place. Next, a gate dielectric 13 such as silicon dioxide or silicon nitride is applied and annealed as necessary. An open contact may be formed, as required, depending upon the particular application. The transistor body layer 14 is applied and this is typically a layer of polysilicon.
In order to form the transistor body from the transistor body layer, a mask 15 is lithographically applied to define the channel, the source and the drain (not shown), as required. The unmasked portion of the body layer is then etched and the mask is removed and the wafer cleaned. This provides the transistor body 16 above the gate electrode 12 separated by the gate dielectric 13. A screening oxide layer 17 is then applied. One can then implant the thin film transistor channel, dope the drain and the source, as required for the particular application.
In applying and removing the photoresist mask, etching the polysilicon layer, and subsequent cleaning of the polysilicon layer, the polysilicon transistor body is contaminated. Applying, curing and subsequently removing the photoresist layer can cause impurities to migrate onto the surface or into the polysilicon layer. Mobile ions present in the photoresist material and etchant can alter the threshold of the transistor. Further, boron from the air can alter the threshold of the transistor. This also affects the useful life of the transistor. In the past, this degradation of the transistor body was either accepted or somewhat reduced by further cleaning the transistor body. But there was no good way to prevent such problems.